x86/ibt: Make cfi_bhi a constant for FINEIBT_BHI=n

x86/ibt: Make cfi_bhi a constant for FINEIBT_BHI=n

Robot yielded a .config that tripped:

  vmlinux.o: warning: objtool: do_jit+0x276: relocation to !ENDBR: .noinstr.text+0x6a60

This is the result of using __bhi_args[1] in unreachable code; make
sure the compiler is able to determine this is unreachable and trigger
DCE.

Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250303094911.GL5880@noisy.programming.kicks-ass.net